For six straight years, the radio telescope at Effelsberg recorded the "breathing" of the distant blazar 1156+295 — a galaxy whose core shoots out a jet of plasma at nearly the speed of light. Every few months, its radio waves flared up and then faded. Astronomers took readings at several frequencies at once, like a doctor listening to a heart not with one stethoscope but with a whole battery of sensors.
It turned out that before each radio flare, the magnetic field in the jet grew stronger first. Only later, with a delay, did the ejection follow — as if a cork were building up elasticity and finally shot out. This rhythm matched the prediction of the "shock-in-jet" model: a blob of plasma flying down the channel sweeps up magnetic field lines ahead of it like a bulldozer pushing snow, and at some point the field becomes so dense that it can't be pushed any further.
This threshold is called the "magnetically arrested disk" regime. Once reached, the growing black hole limits itself: excess gas and dust fly away with the jet instead of falling in. So the giant at the galaxy's center doesn't devour everything around it but leaves building material for new stars. Observations of 1156+295 linked, for the first time, the growth of the magnetic field, changes in the jet's shape, and radio flares — showing that the jet is driven by the magnetic field, not something else.
Now astronomers have a ruler to measure other active galaxies. If their jet magnetic fields also hit this limit, then the scenario is universal: supermassive black holes across the universe are all whistling the same magnetic tune.
Blazar 1156+295 is so far away that its light took 7.3 billion years to reach us — more than half the age of the universe. When that light set off, our solar system had not yet formed.
In Robert Wilson's novel Spin, a black hole at the center of a galaxy controls an entire civilization through jets — like a conductor leading an orchestra. It seems real blazars follow the same score, just without the conductor.